1. Field of the Invention
The present invention relates to a photovoltaic cell including a porous semiconductor layer, a method of manufacturing the same and a solar cell. More specifically, the present invention relates to a photovoltaic cell including a highly efficient porous semiconductor layer which makes use of a photoelectric conversion function of an organic material and is manufactured at low cost, a method of manufacturing the same, as well as a solar cell.
2. Description of Related Art
In connection with global-warming issues, solar cells capable of converting sunlight into electric power have recently received attention as an energy source alternative to fossil fuels. At present, some of solar cells utilizing a crystalline silicon substrate or an amorphous silicon thin film have been put into practical use. However, the former involves high cost for manufacturing the silicon substrate, while the latter requires a variety of semiconductor gases and a complicated manufacturing apparatus including vacuum equipment. That is, even now the cost for manufacturing the solar cells is high and there are problems unsolved.
On the other hand, a dye-sensitized solar cell is gaining the spotlight for its higher photoelectric conversion efficiency as compared with other organic solar cells. The dye-sensitized solar cell uses as a photoelectric conversion material a semiconductor layer on which a spectrum sensitive dye for absorbing light in the visible region is adsorbed (hereinafter, a dye functioning as a photosensitizer is simply referred to as a “dye”). This solar cell has been proposed as a low-cost solar cell.
For example, a dye-sensitized solar cell to which photoinduced electron transfer in a metal complex is applied has been proposed by Gratzel, et al. (see Japanese Patent Gazette No. 2664194, J. Am. Chem. Soc., 115 (1993) 6382, Nature, 353 (1991) 737). The dye-sensitized solar cell is comprised of two glass substrates each carrying an electrode and a photoelectric conversion layer and a charge transport layer which are provided between the electrodes. The photoelectric conversion layer is a porous semiconductor layer on which a photosensitive dye is adsorbed (e.g., a TiO2 thin film). An absorption spectrum thereof lies in the visible region.
Japanese Patent No. 2664194 describes a dye-sensitized solar cell using a metal oxide semiconductor on which a dye made of a transition metal complex is adsorbed.
Here, an explanation is given of manufacturing steps of a common dye-sensitized solar cell.
First, a transparent conductive layer is formed on a surface of a transparent support. A porous semiconductor layer made of titanium oxide is formed on the transparent conductive layer and a dye is adsorbed thereon. Then, a counter electrode coated with a catalyst such as platinum is stacked on the transparent support so that the porous semiconductor layer and platinum are faced to each other. Then, an electrolyte solution which serves as a charge transport layer is injected between the transparent support and the counter electrode and the sides of the transparent support and the counter electrode are sealed with an epoxy resin.
The porous semiconductor layer is formed by coating a suspension containing semiconductor particles on the support, followed by drying and baking at high temperature. The suspension is prepared by adding 4 ml of water and 0.4 ml of acetylacetone to 12 g of fine particles of titanium oxide (P-25 manufactured by Degussa), dispersing the particles in a mortar, diluting the resulting solution with 16 ml of water and adding 0.2 ml of Triton X-100 manufactured by Aldrich as described in J. Am. Chem. Soc. 1993, 115, pp. 6382–6390.
The titanium oxide layer made by using the suspension takes a porous structure. Therefore, the dye is supported in a large amount and a photocurrent value increases. If a polymer such as polyethylene glycol is added to the suspension, the resulting titanium oxide layer takes higher porosity and the dye is supported in a larger amount.
When the photoelectric conversion layer of the dye-sensitized solar cell is irradiated with light, electrons are generated, which are transferred to a counter electrode through an external electric circuit. The electrons transferred to the counter electrode are carried by ions of the charge transport layer and return to the photoelectric conversion layer. Electric energy is generated in such a manner.
Taking such an operation principle into consideration, various attempts have been made with a view to achieving high photoelectric conversion efficiency. In general, improvement in short circuit current density (Jsc) is an important factor for enhancing the photoelectric conversion efficiency of the solar cell. As the porous semiconductor layer in charge of photoelectric conversion, oxide semiconductors such as TiO2, ZnO and SnO2 are considered. It has been reported that a thin film of anatase-type TiO2 which is excellent in photocatalytic effect shows the highest photoelectric conversion efficiency. Under these circumstances, there have been attempted (i) development of a photosensitive dye which absorbs a large amount of light, (ii) control of the particle diameter of the semiconductor particles of the porous semiconductor layer and (iii) increase in film thickness of the porous semiconductor layer with a view to improving the Jsc.
However, for the above (i), a photosensitive dye which is superior to Ru dyes reported as effective in the early times has not been developed though intense researches have been made into organic dyes and metal complex dyes.
For the above (ii), the control of the particle diameter of the semiconductor particles of the porous semiconductor layer is intended to increase the amount of the photosensitive dye adsorbed on the porous semiconductor layer and improve the Jsc. For example, Japanese Unexamined Patent Publication No. 2001-76772 discloses a technique for the control. According to the technique, hollow particles of metal oxide having an average particle diameter of 200 nm to 10 μm are contained in the porous semiconductor layer, thereby providing an oxide semiconductor electrode capable of adsorbing the photosensitive dye and diffusing the charge transport layer sufficiently and easily.
However, even if such hollow particles are contained, there is a limit to the amount of the adsorbed photosensitive dye per unit area of the semiconductor layer. Therefore, there has been no other means of improving the Jsc sufficiently than increasing the film thickness of the porous semiconductor layer.
For the above (iii), if the porous semiconductor layer is thickened, it adsorbs a larger amount of the photosensitive dye and absorbs a larger amount of light. However, electric resistance in the porous semiconductor layer and contact resistance at an interface between the semiconductor electrode and the photosensitive dye increase. That is, even if the porous semiconductor layer is thickened with a view to improving the Jsc, a fill factor (FF) is reduced and hence there is posed a limit in effectively converting light into electric energy. Therefore, it has been difficult to achieve high photoelectric conversion efficiency (Effi).
According to the prior art, dye-sensitized solar cells having the porous semiconductor layers of varying thicknesses were formed. They showed a monotonous increase in Jsc in response to the increase in thickness as shown in FIG. 12. However, the FF was reduced in response to the increase of the Jsc. Accordingly, it was recognized that the conversion efficiency does not increase so much as the Jsc does in response to the increase in thickness (see Comparative Example 1).
From the viewpoint of practical use of the dye-sensitized solar cell, it is necessary, not only to improve the conversion efficiency, but to evaluate and control the properties of the anode electrode so that the so solar cell can be manufactured with stability and high yield. However, there has not been established a simple and effective method of evaluating the anode electrode, which presents a problem in industrialization.
The porosity of the porous semiconductor layer has been evaluated by a specific surface area or a surface roughness coefficient which is described in Japanese Patent Gazette No. 2664194. The “surface roughness coefficient” mentioned herein signifies the ratio between an actual surface area (i.e., an effective surface area) and a projected area of an actual surface of a certain substance.
For the adsorption of the dye on the porous semiconductor layer, the porous semiconductor layer having a thickness of about several μm to several tens of μm is immersed in a solution of the dye dissolved in an organic solvent such as ethanol. Accordingly, even if the porous semiconductor layer actually has a specific surface area or a surface roughness coefficient which seems to allow sufficient adsorption of the dye, dye molecules cannot permeate into the inside of the porous semiconductor layer unless the porous semiconductor layer has a pore radius and pore volume which allow easy permeation of the dye dissolved in the solution. Therefore, there has been a problem in that the dye cannot be adsorbed sufficiently.